In the field of construction machines such as hydraulic excavators and wheel loaders, development of a hydraulic closed circuit is in progress, as a circuit which connects a bidirectionally tiltable hydraulic pump and a hydraulic actuator in a closed circuit form, feeds to the hydraulic actuator hydraulic working oil delivered from the bidirectionally tiltable hydraulic pump, and returns oil having returned from the hydraulic actuator to the bidirectionally tiltable hydraulic pump. A system which includes the hydraulic closed circuit controls a delivery rate of the bidirectionally tiltable hydraulic pump to control a driving speed of the hydraulic actuator.
Examples of this type of system equipped with this type of hydraulic closed circuit include a hydraulic drive system provided to increase a lowering speed of a work implement without using a large-capacity hydraulic pump (see Patent Document 1). A hydraulic drive system described in Patent Document 1 includes a hydraulic closed circuit constituted by a bidirectionally tiltable hydraulic pump and a hydraulic cylinder connected with each other via a hydraulic working oil flow path, and discharges a part of hydraulic working oil discharged from the hydraulic cylinder into a bleed-off flow path branched from the hydraulic working oil flow path without returning the part of the hydraulic working oil to the hydraulic pump at the time of high-speed lowering of the work implement by a pulling action of the hydraulic cylinder.